There are few people as influential in development policy at the moment as Nancy Birdsall. Many of the UK government's key policies have her fingerprints on them as the UK development secretary, Andrew Mitchell, has acknowledged. Indeed, such is her influence that various commentators have referred to the Birdsall effect; once she picks up an idea, it has a curious way of finding its way into policy on one side or the other of the Atlantic.

"Cash on delivery", for example, was an idea she has passionately advocated and which has been picked up by Mitchell; the basic concept being that aid should be tied to clearcut results agreed at the outset, such as numbers of children vaccinated or educated.

First, I liked her modesty. She is not saying all aid should be cash on delivery. And she is certainly not saying that it is a panacea that will transform aid. She is simply saying that it is a tool alongside plenty of others. As a policy, perhaps cash on delivery aid has suffered from over-promotion by politicians eager to present their arrival in office as a fresh start.

Secondly, I thought it was convincing how she emphasised cash on delivery as a mechanism to improve accountability with those populations receiving the aid. This is a perennial problem in the sector: that the accountability runs back to the donors rather than to the people in the recipient country itself. When aid is given for education or water, do the people themselves see the benefits – and if not, why not? How can they call their governments to account for how aid is disbursed?

Birdsall argued that if a donor has made an agreement with a government for cash on delivery with a specific outcome, for example to educate 4 million children, this can help civil society and the media in the recipient country keep their government to the commitment. Instead of aid being poured into complex capacity-building programmes – teacher training, school building, curriculum development – funds are put into something very concrete and simple. At the end of an agreed period, everyone will want to know whether 4 million children are in school and if not, why not.

It sounds as if cash on delivery is as much about communication as aid effectiveness – simply a different way of describing a similar thing. Debt-relief deals with Uganda in the early part of the last decade didn't specify the number of kids who would end up in school but you could have described and structured them in that way.

What difference on the ground does it make? Or is it just that the press release is written in a different way? Is it partly that it makes it easier to sell the aid package to the donor's electorate as well? It sounds better if you talk of millions of kids in school rather than lots of money spent on teacher training.

Birdsall maintained that it also gave an incentive to measure impact. "We don't know how effective aid policies are," she said. This is another issue which has bedevilled aid – the evaluation gap – that there is still only a hazy idea of what works.

Certainly Birdsall's championing of cash on delivery and its take-up by Mitchell is prompting considerable debate in the UK now. There are many advocates who see its accessibility as crucial to improving the credibility of aid. I get their point, but I'm still sceptical that this is a bandwagon which is at risk of acquiring too many passengers. Given Mitchell's attachment to it, anyone keen to have influence on Dfid has to sign up.

"The origins of the results agenda lies in a mistrust that eats like a cancer into aid agencies' capacity to make a difference. I am not convinced the emphasis on results will solve the problem of trust. On the contrary, it risks making things worse. The results rhetoric gets exaggerated by bureaucratic systems and by those middle-level managers with little country-level experience who are forcing grantees and development partners into straitjackets that constrain them from helping transform the lives of people in poverty."

"A results agenda, as long as the right results are being pursued, can help to rebalance inequalities of power and make the actions and decisions of the powerful more transparent. It helps people to know what the objectives of decision-makers are – and so to argue that they should be different, if that's the case; and also to hold people to account for their success or failure to meet those objectives. Without measurement, there can be no accountability."

Hmm – we all know that measurement is a far from neutral tool: who measures what and how they do it becomes a deeply political process.

If you're feeling confused then look at this blog and weep: it cites the UN's assessment of the entire field of "results-based management" used in its agencies:

"Many of the results planned for have been expressed in a self-serving manner, lack credible methods for verification and involve reporting based on subjective judgement."

And:

"The formalistic approach to codifying how to achieve outcomes, inherent to RBM, can stifle the innovation and flexibility required to achieve those outcomes."

It concludes that results-based management "in the United Nations has been an administrative chore of little value to accountability and decision-making…"

The blogpost says that "even some of the major donors who support them [results based management] have admitted that: 'we don't pretend these things describe reality'."